(Adapted from the application) One approach to Alzheimer's disease therapy is inhibiting fiber formation. If efficacious therapeutic agents are to be developed, the molecular factors controlling fibril nucleation, fibril elongation, and fiber-fiber association must be identified. The applicant's have developed a powerful new paradigm for achieving this goal. Key elements of this paradigm include: 1) a model system for the reproducible growth of A beta fibers; 2) the use of quasielastic light scattering spectroscopy (QLS) to continuously, quantitatively monitor all phases of fibril growth, including prenucleation and nucleation, fiber elongation, and fiber-fiber association; and 3) a mathematical framework which operates upon the QLS data and provides numerical values for the parameters describing each phase of the fibrillogenesis process, including rates of nucleation and elongation. They propose to use this paradigm: 1) To determine the elements in the primary structure of A beta that control the kinetics of fibrillogenesis. They will study the kinetics displayed by wild type A beta peptides, by elongated and truncated A beta peptides, and by A beta molecules containing Glu22>Gln (Dutch), AIa21>GIy (Flemish), and Phe19>Pro substitutions. 2) To determine the effects of the biochemical milieu on the kinetics of A beta fibrillogenesis. The role of pH and ionic strength in controlling fibrillogenesis will be examined. To examine the energetics of fibril nucleation and elongation, they will study the temperature dependance of the rate for each process. 3) To determine the effects of AI3 variants and ApoE on the kinetics of wild type A beta fibrillogenesis. Three mixing strategies will be employed to assess the effects of structural variants of A beta, and of ApoE2, ApoEC, and ApoE4, on the nucleation and elongation stages of fibrillogenesis. 4) To determine the mechanisms of action of fibrillogenesis inhibitors. They will establish quantitatively how fibrillogenesis inhibitors affect A beta micellization, nucleation, and elongation. Four classes of inhibitors will be studied: amyloid dyes, peptides, sulfates, and surfactants. Based on initial results from these studies, as well as from the prior three aims and Project 1, systematic changes in the structures of the inhibitors will be made in order to identify the critical molecular elements mediating each type of fibrillogenesis inhibition.